1. Technical Field
Embodiments of the present disclosure generally relate to semiconductor devices and methods of operating the same and, more particularly, to nonvolatile memory devices and methods of operating the same.
2. Related Art
Semiconductor memory devices may be typically categorized as either volatile memory devices or nonvolatile memory devices. The volatile memory devices lose their stored data when their power supplies are interrupted even though their operation speed (e.g., a time it takes to read out data stored in memory cells or a time it takes to write external data into the memory cells) is relatively fast. In contrast, the nonvolatile memory devices retain their stored data even when their power supplies are interrupted. Thus, the nonvolatile memory devices are used in electronic systems that need to retain certain data regardless of interruption of the power supplies. The nonvolatile memory devices may include mask read only memory (MROM) devices, programmable read only memory (PROM) devices, erasable programmable read only memory (EPROM) devices, electrically erasable programmable read only memory (EEPROM) devices, flash memory devices and etc.
In general, the MROM devices, the PROM devices and the EPROM devices may need some additional utilities (e.g., a UV irradiator) to erase their stored data. Thus, it may be inconvenient to operate and use the MROM devices, the PROM devices and the EPROM devices. In contrast, the data stored in the EEPROM devices can be electrically erased even without any additional utilities and external data can also be electrically programmed into the EEPROM devices without any additional utilities. Accordingly, the EEPROM devices may be widely used in various areas, for example, system program executions or auxiliary memory devices necessitating frequent data renewals. In particular, data stored in the flash memory devices can be simultaneously erased by the certain unit (e.g., by the page) and the flash memory devices may have a higher integration density than the EEPROM devices. Hence, the flash memory devices have been used in the auxiliary memory devices having a large capacity.
The number of data, which is capable of storing in each memory cell of the nonvolatile memory devices, may be determined according to the number of bits that are stored in each memory cell. A memory cell in which a single bit data is stored is referred to as a single bit cell or a single level cell (SLC). In contrast, a memory cell in which multi-bit data (e.g., data including at least two bit or more) are stored is referred to as a multi-bit cell, a multi-level cell (MLC) or a multi-state cell. Recently, as the semiconductor memory devices become more highly integrated, the nonvolatile memory devices employing the MLCs have been increasingly required and developed.